Continuing on forms of direct analysis from Part 10: DART & DESI, the series now moves on to look at headspace as an extraction technique.

Headspace is the classical technique that can extract the sample without the need for a solvent. As such, only volatile-to-semi-volatile species are extracted. Headspace is a means of introducing the volatile components from a liquid or solid sample into a gas chromatograph for analysis when it is difficult or impossible to inject the actual sample. Also, if very volatile species are present and an extracting solvent would interfere with the analysis, headspace offers an analytical methodology. The basic principle is well documented[1] and as such this article will only cover the most basic principles.  The basic principle is as follows: The solid, liquid or gaseous sample is placed in a vial and sealed. The vial is heated, allowing the volatile components to escape out of the sample to form a gaseous headspace. After a set time, the headspace gas is extracted from the vial and injected into a gas chromatograph for analysis. Headspace theory is based around the tendency of a material to go into the gas phase, producing the partition coefficient, K.  K is defined as CL divided by CG (where CL is the concentration of the analyte in the condensed phase and CG is the concentration of the analyte in the gas phase). This is shown schematically in Figure 1. Reducing K will increase the sensitivity of the Headspace analysis.  Methods to reduce K are listed below.

  • Addition of mineral salts to the matrix (sodium sulphate is common)
  • Addition of another liquid to the matrix
  • Increasing the temperature
  • Agitation of the sample

Figure 1 Headspace extraction (schematic)

For extractables, the key factors are temperature and time, especially for solid samples.  The time to diffusion t is related to the diffusion coefficient D and the diffusion path length

t= d2/2D

Ideally, the temperature should be above the glass transition temperature of the material to enhance the rate of species diffusing through a material. In general, Headspace is a simple, reliable and easily automated technique which allows for the analysis of very volatile species without potential interference from an extracting solvent. It does not, however, provide as-low detection limits as other Headspace techniques such as solid-phase microextraction (SPME), in-tube extraction, or dynamic headspace (DHS).  How these related techniques work and their advantages will be discussed in subsequent blogs.

References

[1] Kolb and L.S.Ettre in Static Headspace Gas Chromatography, Wiley-VCH, Inc., New York, NY, USA, 1997.